Removal of zinc from lead



Patented Mar. 21, 1944 REMOVAL .OF ZINC FROM LEAD Walter T. Monson, Newark, N. 1., assignor to The American Metal Company, Limited, New York, N. Y., a corporation of New York No Drawing. Application February 11, 1942,

Serial No. 430,362

4 Claims.

This invention relates to the removal of zinc from lead, especially lead which has been de- 5 silverized and contains only a small percentage of other impurities than zinc.

In the usual of refining lead to produce desilverized lead, one stepconsists. of removing silver by treating with zinc. The silver is removed with most of the zinc as a zinc crust. However, .5 to .6% of. zinc remains dissolved in the lead and this must be removed to make a fully refined lead. Formerly this zinc was removed by pumping the lead into a refining furnace and blowing steam into the metal, whereby a zinc-oxide-lead-oxide slag was formed and skimmed oif. More recently zinc has been. re-

, moved by-treating the lead in a kettle with calcium-hydroxide, sodium-hydroxide or similar chemical and stirring in order to form a dross containing the zinc. In another process zinc has been removed by passing chlorine into the impure lead and forming zinc chloride which was skimmed from the metal bath. All these .methods have required expensive reagents and the cost of removal has been appreciable.

An important object of my invention, accordingly, is to produce a simple and eflicient process for the removal of zinc from desilverized lead at relatively low cost.

The invention accordingly consists of a process comprising treating impure lead containing zinc but substantially free from other impurities by stirring the lead in molten condition at a relatively high speed while maintaining it at a temperature of about 1100-1200" F. in the absence of an alkali-metal oxidizing agent, with an oxidizing agent selected from the group consisting of-air and added powdered litharge, and added powdered-litharge alone, where air and litharge are used as the oxidizing agent, the

. litharge being added substantially at the commenceznent of the stirring operation and constituting a quantity in addition to that formed by contact of the air with the impure lead; and the stirring being conducted under an atmosphere of inert gas when litharge alone is used as the oxidizing agent. .A dross is formed thereby consisting substantially of zinc oxide and lead oxide. I prefer to add the litharge as a slag containing zinc oxide in addition to the litharge or lead oxide.

I have found in accordance with my invention that by stirring the lead-zincbath at highspeed and high-temperature with oxidation, I can quickly remove all but traces of zinc from metal containing .5 to .6% or more of zinc and produce a dross that contains 25% or more zinc content. The basis of my dezincing process is the selective oxidation of the zinc in the de- Y silverized lead bath and the'additional concentration by replacement reactions between lead oxide in the dross and zinc in the lead. The typical replacement reaction is:

PbO Zn 7.110 Pb (Lead oxide plus zinc) (Zinc oxide plus lead) I have operated the process of my invention at speeds of about 900 R P. M. and above and temperatures of 950 F. and above, with apparatus similar to that describedby Harper and Reinberg in U. S. Patent No. 2,155,545 with excellent results. However, in my preferred practice. the temperature is kept over ll00 F. and

the stirring speed about 1700 R. P. M. This'gives a higher rate of zinc removal and a higher grade dross. I

In practising my invention, I prefer to place an oxide cover of zinc-oxide-lead-oxide slag -on the bath substantially at the commencewet at the start, even if a dry dross is added as starting oxide. In order to get einfcient dezinclng, it has been found desirable to add a little rosin and sal ammoniac (ammonium chloride) after which the dross stays dry until dezincing is complete. A cover over the kettle has been found helpful when working with dry drosses to keep dust from passing out into the atmosphere, but the cover is sufiiciently loose to permit entry of air to permit oxidation of further quantities of lead used up in the above replacement reaction.

Desilverized lead contains very small amounts of impurities other than zinc, so that the only two metals available for oxidation are zinc and lead. It is known that lithar'ge formation is very rapid under oxidizing conditions when the drosses are higher in zinc, whereas the later drosses start in to decrease in zinc content and go up in lead content. I have found that if the dross is skimmed after two or more alternate intervals of stirring and skimming, the last dross skimmed may be used on the next charge to be treated for zinc removal with very good results.

I have also found that it is possible with my process to make drosses very high in zinc and low in lead, by stirring a zinc-oxide-lead-oxide dross on a bath of zinc, or metal bath high in zinc content. The dross stirred in may be of the type produced by my process described above, or it may be from some other source. By stirring in at high speed and high temperature a dross containing, say, from around to zinc as oxide and the balance lead as oxide, it has been possible to bring the zinc content up to over 70%.

In employing metallic baths which are high in zinc in order to make a dross with a high zinc-content, I have found that too muchoxidation of the zinc bath occurs by air drawn in at the vortex caused by the impeller unless precautions are takento prevent it. The air thus drawn insets to convert the zinc in the bath to zinc oxide in excess of that which should be formed by replacement reactions with lead oxide. In this operation, it is desirable to get as much replacement reaction between lead oxide in the dross and metallic zinc as possible in order to obtain the greatest economic advantages of the invention. *1 have accordingly overcome the above-mentioned dimculty by operating in a neutral atmosphere to prevent primary oxidation of the zinc. In order to do this, I-seal the kettle with a gas-tight cover having an inlet for a neutral gas and an outlet. A snug-fitting sleeve for the shaft from the motor to the impeller decreases the amount of gas used since the neutral atmosphere on the bath is kept under. a slight pressure in order to insure that no air enters. in my preferred practice, I have found nitrogen or carbon dioxide (CO2) to be very eillcient for this purpose, but other gases which would retard or prevent the formation of zinc oxide due to-atmospheric conditions over the metal bath may also I be used.

In order to better illustrate my process, the following examples are given-but the invention is not to be considered as limited thereto except as indicated in the appended claims.

Example 1 In an apparatus such as that shown in Patent No. 2,155,545, to 7,600 lbs. of lead bullion-containing .776% zinc, the balance being substantially all-lead, was added 595 lbs. of oxide from another kettleassaying 20.45% zinc as oxide and the balance lead as oxide. The 'bath was stirred with access of air for one hour at a speed of 1050, R. P. M. and 1100 1". The lead at the end of thehour contained only a trace of Zinc.

The dress skimmed of! assayed 25.27% Zn.

The metal balance and rate of removal were as follows:

, Example 2 The following example illustrates the method used to increase the zinc content of drosses by stirring a lead-oxide-zinc-oxide dross on a bath of zinc or metal high in zinc employing an apparatus as in Example 1. No attempt was made to exclude air from the zinc bath as was done in Example 3 given below. As a result, the amount of zinc entering the dross for each pound of lead reduced is higher in the present example than in Example 3, as indicated by the details of the runs given in the examples.

To 4,640 lbs. of zinc assaying 98.2% Zn and 1.8% Pb, melted in a kettle, was added 335 lbs. of mixed zinc and lead oxide containing 17.99% zinc and 69.6% lead. The bath while open to the air was stirred for 15 minutes at a speed of 1700 R. P. M. and a temperature of 1140 F.

The dross skimmed of! at the end of the run assayed 69.79% Zn and only 11.8% lead.

Details of the run follow:

For each pound of leadreduced to metal, ,1.6 lbs.

of zinc were transferred to the dross which is more than the theoretical amount of about .3 lb. This is partly due to zinc entrained in the dross and partly due to primary oxidation by air stirred into the bath.

Bramble 3 In order to prevent the primary oxidation of zinc, during the grading up of zinc drosses, an appara like that in Patent No. 2,155,545 was employed but the kettle was covered and a neutral atmosphere was kept over the bath.

In the following example an atmosphere of CO: was kept on the bath during the stirring operations. The pressure of the CO: gas was maintained at one inch of water above atmosphere pressure to prevent any air from entering.

To 4,996 lbs. of zinc assaying 98.8% Zn and 1.2% Pb was added 288 lbs. of zinc oxide-leadoxide dross running 29.07% Zn and 57.60% Pb.

The charge was stirred for 30 minutes under an atmosphere of CO1, except for the last two minutes when 1 lb. of ammonium chloride (1711401) and 2 lbs. of rosin were added to dry the wet zinc dross.

A temperature of 1160 to 1200 F. was maintained and a stirring speed of rzoo a. an. I

At the end of the stirringperiod 316 lbs. of dross were skimmed containing 70.89% Zn and 13.00% Pb. l

The details of the run are as follows:

Momma Weight 5" a? il Madman".-. 4.00s ass 1.2 4,900.0 sac Metllend use ,0 a1 4,78i.3 m1

In this case, 1.1 lbs. of zinc entered the dross for each pound of lead reduced. In Example 2, 1.6 lbs. ofzinc entered the dross for each pound of lead reduced, showing the improvement with the protective atmosphere in the above Example 3.

A 9% inch impeller is used in a 40-ton kettle. For a smaller sized kettle, such as a 40-inch ketle, an impeller of 6 inches may be used. For larger or smaller sized kettles, a variation in the size of the impeller is preferably made, the variation being made in accordance with the dimensions of kettles and impellers mentioned above.

The examples illustrate the invention as I now prefer to practise it, and represent a simple eflicient process for treating zinc and lead to form drosses of zinc oxide and lead oxide. These drosses may be further treated to recover the metal values.

Although I have mentioned rosin and ammonium chloride as materials which are preterably employed for drying the dross, other materials may be used instead, such as oil, grease, sawdust, or powdered coal.

' What I claim is:

1. A process for treating impure lead containing zinc but substantially free from other impurities which comprises stirring the impure lead in molten condition at high speed while maintaining it at a temperature of about 1100- 1200 F. in the absence of an alkali-metal oxidizing agent, with an oxidizing agent selected from the group consisting of air and added powdered litharge, and added powdered litharge alone; where air and litharge are used as the oxidizing agent, the litharge being added substantially at the commencement of the stirring operation and constituting a quantity in addition to that iormed by contact of the air with the impure lead: and the stirring being conducted under an atmosphere of inert gas whe litharge alone is used as the oxidizing agent; thereby forming a dross consisting substantially oi zinc oxide and lead oxide. 5 2. A process for treating an impure lead containing small quantities of zinc but substantially free from other impurities which comprises stirring the impure lead with an oxidizing agent consisting of added powdered litharge and air at about 1100-1200" F. in the absence of an alkalimetal oxidizing agent, the litharge being added substantially at the commencement of the stirring operation and constituting a quantity in addition to thatformed by contact of the air with 5 the impure lead, and thereby increasing the zinc oxide content of the dross so that it contains at .least 25% zinc as oxide. 3. A process for treating an impure lead containing small quantities of zinc but substantially free from other impurities which comprises stirring the. impure 'lead with an oxidizing agent consisting of added powdered litharge at about 1100-1200 F. in the absence of an alkali-metal oxidizing agent, the stirring being conducted under an atmosphere oi inert gas, thereby increasing the zinc oxide content of the dross so that it contains at least 25% zinc as oxide. 4. A process for treating .a mixture of zinc and lead which comprises placing a layer of dross consisting of zinc oxide and lead oxide on a bath of zinc and lead in which bath there is a relatively small proportion of lead and stirring the bath and dross layer at about 1100 1200 F. while maintaining an inert atmosphere above e surface of the bath and dross layer, thereb increasing the content of zinc oxide in WALTER. T. MONSON.

OER'HFIGA'I'E 0F commends.

Patent No. 2 1 1 529. March 21, 191 1;.

warms T. HONSON.

It is hereby certiffied-that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, first column, line 5, for "ketle' read "kettleand second column, line 56, claini 1;, after "in' insert --the dross layer.--; and that the saidI-etters Patent should be read with this correction therein that the same may conform to the record of theucaae in the Patent Office.

Signed and sealed this 16th dsy of "may, A. D. 191 1;,

Lesliei Frazer (Seal) Acting Commissioner of Patents. 

